Inexpensive sources of purified hydrogen are sought after for many industrial chemical production processes and in the production of energy in fuel cell power systems. Similarly, inexpensive methods of purifying hydrogen could significantly expand the applicability of hydrocarbon reforming, reforming reactors and the water gas shift reaction.
Palladium and its alloys, as well as nickel, platinum and the metals in Groups III-V of the Periodic Table are all permeable to hydrogen. Hydrogen-permeable metal membranes made of palladium and its alloys are the most widely studied due to their high hydrogen permeability, their chemical compatibility with many hydrocarbon containing gas streams, and their theoretically infinite hydrogen selectivity. Hydrogen molecules (H2) present in a steam of mixed gas molecules will dissociate into hydrogen atoms, which dissolve into the palladium and diffuse across a palladium metal barrier to recombine into hydrogen molecules and dissociate from the opposite surface of the palladium barrier as a purified hydrogen gas. Thus, a gas stream formed in different industrial processes that contains many different molecular components including hydrogen can be directed to a palladium membrane to selectively recover the hydrogen present in the gas, thereby producing a purified hydrogen gas stream without significant additional energy input.
Unfortunately, pure palladium membranes are themselves expensive when used in such purification processes due to their rapid degeneration and limited life. Atomic hydrogen is so soluble in palladium that it forms a separate hydride phase (β), which has a much larger lattice constant, causing swelling, warping and cracking of the palladium membrane. This α←→β phase transition takes place at the critical temperature of 295° C., making it difficult to avoid premature breakdown during prolonged industrial use. Additionally, sources of sulfur, present in many industrial process gasses, produce hydrogen sulfide when they contact palladium membranes at high temperature. Hydrogen sulfide is a potent poison of the hydrogen dissociation catalysts including palladium metal membranes, and exposure to sulfur-bearing gasses rapidly lowers the permeability of a palladium membrane to hydrogen requiring the replacement of the relatively expensive membrane structure.
In an attempt to overcome these problems with pure palladium membranes, alloys of palladium have been tested that display a comparable hydrogen permeability with superior physical strength and greater resistance to thermal degradation and sulfur poisoning. As early as 1963, McKinley (U.S. Pat. No. 3,350,845) formulated alloys of palladium with copper, silver and gold and showed that palladium-gold alloys containing about 55 weight percent gold had improved resistance to poisoning by sulfur-containing gases, albeit with about a 10-fold decrease in hydrogen permeability. Alternatively, palladium-silver membranes and palladium-copper membranes containing about 10 weight percent silver and about 40 weight percent copper, respectively, showed an increased permeability to hydrogen but were equally or more sensitive to sulfur poisoning compared to pure palladium membranes.
The palladium-gold membranes disclosed by Mckinley were relatively thick and prepared by conventional metallurgy techniques. Such membranes are still prohibitively expensive for most industrial applications. Therefore, there has been a long-felt need for a fabrication method capable of inexpensively and efficiently producing palladium alloy membranes having high thermal stability, durability and resistance to sulfide poisoning.
Recent research efforts have focused on the development of composite metal membranes consisting of a relatively thin Pd or Pd-alloy coatings on hydrogen permeable base metals, or on porous ceramic or stainless steel supports.
Many palladium alloys such as Pd73Ag27, Pd95Au5, and Pd60CU40 possess higher hydrogen permeability than pure palladium. In the 1960s, McKinley and coworkers (U.S. Pat. No. 3,439,474 (1969)) reported that binary alloys of Pd with Au and Cu had pure hydrogen permeabilities greater than Pd and PdAg, were unaffected by thermal cycling, and had some resistance to sulfur poisoning by hydrogen sulfide. The inhibition or reduction of the pure hydrogen flux due to exposure to 4 ppm hydrogen sulfide through the 40 mass percent Au alloy was the least compared to pure Pd, PdAg and PdCu alloys.
The sulfur resistance of PdCu foil membranes was investigated by researchers at the DOE NETL laboratory (B. D. Morreale, B. D, et al., J. Membr. Sci., 241:219 (2004)). They reported the best sulfur resistance with a 20% Cu in Pd binary alloy having an FCC crystal structure. But this Pd80Cu20 alloy has only 20% of the hydrogen permeability of pure Pd and about 2 times less than 40% Au.
Thus, there is still a need for sulfur resistant, composite metal membranes and improved methods of designing and making these membranes.